Maintenance in isolation of: Approximately 5,000 inbred female mice with a relatively high content of murine mammary tumor virus in their milk; Five low mammary tumor incidence inbred mouse strains of approximately 500 animals; and 2,500 mice as a source of spontaneous mammary tumors. Contractor will skim, concentrate by ultracentrifugation, and purify type B virus by combined zonal and isopycnic centrifugation in density gradients. In addition: Quantify by specific immunoassays and electron microscopy content of murine type B particles; Perform assays for protein and RNA dependent DNA polymerase; Tissue culture evaluation, storage, and shipment; and Purify virus preparations with precise accountability.